Plasmacytoid dendritic cells (pDCs) represent a unique immune cell lineage that plays an essential role in innate immune responses against viruses. pDCs recognize virus-associated nucleic acids and respond by rapidly secreting massive amounts of type I interferons (IFN), hence their other name: type I interferon- producing cells (IPCs). pDCs can also differentiate, upon activation, into conventional dendritic cells that are capable of presenting antigens to the adaptive arm of the immune system. Thus, pDC are critical for the body's control of viral infections. On the other hand, abnormal pDC hyperactivity has been associated with increased type I IFN levels in autoimmune diseases such as psoriasis and systemic lupus erythematosus. Therefore, pDCs do play important roles in both normal and aberrant immune responses and represent potential useful targets in the treatment of some diseases. pDCs possess unique cellular and molecular characteristics and gene expression profiles that enable them to directly recognize viruses, secrete massive amounts of type I IFN, and differentiate into conventional dendritic cells. Over the last few years, these molecular and cellular features of pDCs have been well characterized;however, the genetic basis of pDC lineage commitment, maturation, and function still remains poorly understood. The overall objective of our research project is to study the role of basic helix-loop-helix transcription factors in the development and function pDCs, and elucidate the pathways that these transcription factors control in pDCs. PUBLIC HEALTH RELEVANCE: These studies will give us insights into the genetic network which controls the development and function of pDC, and may pave the way for the development of new approaches in the treatment of viral infections and some autoimmune disease.